1. Technical Field
The present disclosure relates to a semiconductor memory device and, more particularly, to a test mode semiconductor memory device in which in a test mode of N steps each test item can be individually reset, thereby reducing a test time and providing testing convenience.
2. Discussion of the Related Art
A test for detecting a particular defect in a semiconductor memory device is performed not in a normal mode of the memory but in a particular test mode which can best detect a corresponding defect. To this end, the semiconductor memory device can have a test mode register therein for setting a test mode of the semiconductor memory device.
The test mode register programs and stores various options such as a column address strobe (CAS) latency, a burst type, and a burst length which respectively correspond to a plurality of different test modes. When a command for setting a certain test mode is externally inputted, the test mode register perceives a test mode corresponding to the inputted command and generates a test mode setting signal for making the semiconductor memory device set to the perceived test mode.
A conventional semiconductor memory device is described below with reference to the accompanying drawings.
FIG. 1 is a block diagram illustrating a conventional test mode semiconductor memory device. The test mode semiconductor memory device of FIG. 1 includes a control signal generator 10, a latch 11, a row address latch 12, a column address latch 13, a column decoder 14, a row decoder 15, a memory cell array 16, a sense amplifier 17, an output buffer 18, a pad 19, a set/reset signal generator 20, a test control signal generator 30, and a test logic portion 40.
The control signal generator 10 receives commands from an external portion of the test set up and outputs various control signals to the column decoder 14, the row decoder 15, and the test logic portion 40. The latch 11 receives mode setting signals from the external portion via an address bus and transmits them to the set/reset signal generator 20 and the test logic portion 40. The row address latch 12 and the column address latch 13 receive the mode setting signals A[12:0] from the external portion and transmit them to the row decoder 15 and the column decoder 14, respectively. The column decoder 14 and the row decoder 15 are connected to the memory cell array 16, and an output of the memory cell array 16 is sequentially transmitted to the sense amplifier 17, the output buffer 18, and the pad 19.
The set/reset signal generator 20 receives a test signal TMRS from the control signal generator 10 and mode setting signals MA[12:0] from the latch 11 to output a test MRS reset signal TMRS RESET. The test logic portion 40 receives the mode setting signals MA[12:0] from the latch 11 to output N-bit set signals LS[8:0], where L is 1 to N. The test control signal generator 30 receives the set signals LS[8:0], where L is 1 to N, from the test logic portion 40 and the test MRS reset signal TMRS RESET from the set/reset signal generator 20 to output and apply test control signals TMRS SET0 to TMRS SETM to all item test target blocks of the semiconductor memory device.
Operation of the conventional test mode semiconductor memory device is described below with reference to FIG. 1.
When the control signal generator 10 receives various commands from the external portion to output signals, including a clock signal CLK and a test signal TMRS, for controlling an internal operation of the semiconductor memory device and the latch 11 receives the mode setting signals A[12:0] via an address bus from an external bus and outputs them, the test logic portion 40 receives the clock signal CLK and the test signal TMRS from the control signal generator 10 and a combination of the mode setting signals MA[12:0] from the latch 11 to recognize entry into the test mode, the test logic portion 40 outputs the N-bit set signals LS[8:0], where L is 1 to N, of first to N-th steps MRSes. The set/reset signal generator 20 receives the mode setting signals MA[12:0] from the latch 11 to output the MRS reset signal TMRS RESET according to a combination of the mode setting signals, as defined by the original circuit designer. The test control signal generator 30 receives the first to N-th step set signals LS[8:0], where L is 1 to N, from the test logic portion 40 and decodes them to select a certain test mode MRS unit before outputting the test control signals TMRS SET0 to TMRS SETM of the corresponding items. The test control signals TMRS SET0 to TMRS SETM are individually applied to the row decoder 15, the column decoder 14, the sense amplifier 17, the output buffer 18, and the pad 19, which are internal blocks of the semiconductor memory device, in order to control the set or reset of a test MRS
FIG. 2 is a block diagram illustrating the test control signal generator 30 of the conventional test mode semiconductor memory device shown in FIG. 1. The test control signal generator of FIG. 2 includes a plurality of test mode MRS units 30-1 to 30-M.
A combination of the first to N-th step set signals LS[8:0], where L is 1 to N, is applied to an input terminal of each test mode MRS unit 30-I . . . 30-M from the test logic portion 40, and the test MRS reset signal TMRS RESET is applied to a control terminal of each test mode MRS unit 30-I . . . 30-M from the set/reset signal generator 20, so that the test control signals TMRS SET 0 to TMRS SET M are outputted from an output terminal of each test mode MRS unit 30-I . . . 30-M. That is, a combination of zero-th bits 1S[0] to NS[0] of the first to N-th set signals LS[8:0], where L is 1 to N, is applied to the input terminal of the first test mode MRS unit 30-1, and in the same way a combination of 8-th bits 1S[8] to NS[8] of the first to N-th set signals LS[8:0], where L is 1 to N, is applied to the input terminal of the M-th test mode MRS unit 30-M. Here, M is a number of cases of 512, which is 2 to the 9th power and which is a combination of the 9-bit mode setting signals MA[12:8] and MA[3:0] except for bits MA[7:4], for a safety key for entering the test mode. But, it is possible to make a number of cases of 2 to a higher power by increasing the bit number of the mode setting signals according to a need of the circuit designer.
First, the test MRS reset signal TMRS RESET from the test logic portion 40 is applied to the control terminal of the first test mode MRS unit 30-1, so that the first test control signal TMRS RESET 0 is outputted from the output terminal of the first test mode MRS unit 30-1 by a combination of the zero-th bits 1S[0] to NS[0] of the first to N-th step set signals LS[8:0], where L is 1 to N, and control of the test MRS reset signal TMRS RESET. In the same way, the test MRS reset signal TMRS RESET is commonly applied to the control terminal of the second to M-th test mode MRS units 30-2 to 30-M from the set/reset signal generator 20, so that the second to M-th test control signal TMRS RESET 1 to TMRS RESET M are outputted from the output terminal of the second to M-th test mode MRS unit 30-2 to 30-M by a combination of the first to eighth bits of the first to N-th step set signals LS[8:0], where L is 1 to N and control of the test MRS reset signal TMRS RESET.
FIG. 3 is a circuit diagram illustrating the conventional N-step test mode MRS unit 30-M. The N-step test mode MRS unit of FIG. 3 includes a NAND gate NAND, a PMOS transistor PMOS, an NMOS transistor NMOS, a transmission gate TG, and four inverters INV1 to INV4. The first to N-th step set signals LS[8:0], where L is 1 to N, are applied to the NAND gate NAND, an inverted test MRS reset signal TMRS RESET is applied to a gate of the PMOS transistor PMOS, and a power voltage VDD is applied to a gate of the NMOS transistor NMOS. An output of the NAND gate NAND is applied to a gate of the PMOS transistor of the transmission gate TG, and an inverted output of the NAND gate NAND is applied to a gate of the NMOS transistor of the transmission gate TG. A drain of the NMOS transistor NMOS is applied to one side of the transmission gate TG, and the output of the transmission gate TG is inverted by the inverter INV2 and then outputted as the test control signal TMRS SET. A power voltage is applied to a source of the PMOS transistor PMOS, and a drain of the PMOS transistor is connected to the other side of the transmission gate TG and the inverted output of the fed back test control signal TMRS SET.
Here, the first to N-th step set signals LS[8:0], where L is 1 to N, are decoding signals to prevent the chip size from being increased because the chip size is increased in increments of connection wires for connecting selected lines to the actual circuit when the test MRS unit selects each item to add a test MRS item.
An operation of the conventional N-th test mode MRS unit 30-1 . . . 30-M circuit is described below with reference to FIG. 3.
In FIG. 3, if the test MRS reset signal TMRS RESET is applied with a low level, it is inverted to a high level by the fourth inverter INV4 and then applied to the gate of the PMOS transistor PMOS. The PMOS transistor PMOS is turned off, and the test control signal TRMS SET maintains a low level. However, if all of the first to N-th step set signals LS[8:0], where L is 1 to N applied to the NAND gate NAND have a high level, the NAND gate NAND outputs a low level, and a low level of a ground voltage passing through the NMOS transistor NMOS passes through the transmission gate TG, and so the test control signal TMRS SET having a high level is finally outputted, whereby a test MRS item is applied. If even one of the N input signals has a low level, the NAND gate NAND outputs a high level, and a low level passing through the NMOS transistor NMOS does not pass through the transmission gate TG, and the test control signal TMRS SET is fed back through the third inverter INV3 and then inverted again by the second inverter INV2, whereby the test control signal TMRS SET is maintained to a high level. Meanwhile, if the test MRS reset signal TMRS RESET having a high level is applied, it is inverted to a low level by the fourth inverter INV4 and then applied to the gate of the PMOS transistor PMOS. The PMOS transistor is turned on, and the power voltage VDD passes through the PMOS transistor PMOS and the second inverter INV2, so that the test control signal TMRS SET having a low level is outputted.
FIG. 4 is a timing diagram illustrating an N-step test mode operation of the conventional semiconductor memory device. FIG. 4 shows a timing diagram of signals such as a clock signal CLK, a clock enable signal CLE, a chip select bar signal /CS, a row address strobe bar signal /RAS, a column address strobe bar signal /CAS, a write enable bar signal /WE, a bank address bar signal /BA[1:0], and mode setting signals AO through A12 represented herein by the notation A[12:0]. The clock signal CLK is toggled by sequentially repeating a low level and a high level, the clock enable signal CLE has a high level, and the signals /CS, /RAS, /CAS, /WE, and /BA[1:0] have a low level, and a test mode is entered by a combination of the commands and the mode setting signals A[12:0], that is, by setting “1000” to A[7:4] and loading values of the mode setting signals to A[3:0] and A[12:9] during five cycles. Here, A[7:4] are bits which are not frequently used, and since it functions as a password for preventing easy entry into the test mode to set “1000” as the safety key for allowing entry into the test mode from a normal mode, the circuit designer can, use a combination of different bits.
In FIG. 4, all operations are initiated when the clock signal CLK goes high. In order to apply a first test mode MRS item, the mode setting signals A[3:0] and A[12:8] load data of 1S0 to 1S8 of the first step, 2S0 to 2S8 of the second step, and NS0 to NS8 of the N-th step, latches the data after a predetermined time lapses and then deviates from a latch cycle. In order to apply a second test mode MRS item, the mode setting signals A[3:0] and A[12:8] load data of 1S0 to 1S8 of the first step, 2S0 to 2S8 of the second step, and NS0 to NS8 of the N-th step again, and latches the data like when the first test mode MRS item is applied. During an operation of the test mode operation, the test mode can be reset, if needed, by setting of the mode register provided in a specification table of a data sheet, for example, by setting a value of A[7] to “0”.
However, if the test mode for an individual item is reset by the above described method, all of the test mode MRS items are reset. For the foregoing reason, even when a reset is needed for a re-experimentation or retest of a certain item, the test mode should be inevitably initiated again after resetting all of the test mode MRS items as represented by values m0 through m12 in FIG. 4. That is, a reset for an individual item is impossible because the reset signal of the conventional N-step test mode is shared by all of the test MRS set units, and entry into the test mode is also canceled when the test mode is reset because the reset signal of the test mode register is generated by the normal mode register set.
For the foregoing reasons, since the test using the test MRS set applies the test mode MRS reset just once, the test mode must be entered again in order to carry out the experimentation for another test item. For the experimentation in which simultaneous application of the test mode MRS items is impossible or a combination of a plurality of test mode items is needed, “test mode entry”, “test mode item application”, and “test mode release” should be innumerably and repetitively performed. A finite time for entry of the test MRS set and application of an individual item is needed, and so such repetition in every experimentation operation is extremely time-consuming.